Ion-conducting membranes, including ionomeric membranes such as Nafion™, are an important component in membrane separation processes and electrochemical reactor systems including chlor-alkali cells, electrolysis cells and fuel cells. Such membranes act as ion conductors while preventing reactants from inter-mixing. In some applications, the ions conducted by such membranes are protons. The availability of materials which are solid and can conduct protons has allowed a breakthrough in the production of simple and robust fuel cell devices.
In typical prior art fuel cells, the ion-conducting membrane is an ionomeric membrane that fulfills several functions including providing ion conductivity, providing a barrier between reactants and providing a structural spacer that withstands the clamping forces necessary to seal the fuel cell.
The design of ion-conducting membranes for use in electrochemical cells typically requires balancing between two competing design objectives. Firstly, it is generally desirable to maximize the conductivity of the ion-conducting membrane to minimize operational losses. This first objective tends to favor ion-conducting materials which have high water contents and therefore approach liquid form. Secondly, it is generally desirable to provide a membrane that is robust and usable as a structural material within the cell to maintain integrity of the cell in the presence of differential pressures across the membrane. This second objective tends to favor ion-conducting materials which are solid and have high strength. It will be appreciated that these two design objectives often conflict with one another. Current practices for designing electrochemical cells involve making compromises between these design objectives.
An example of an ion-conducting material is Nafion™, which is typically provided in the form of sheets that may be as thin as 25 microns. FIG. 1 is a schematic cross-sectional view of a Nafion™ membrane 8. Membrane 8 is a continuous sheet of ion-conducting material. Nafion™ membranes are susceptible to mechanical failure and are difficult to work with, especially if they are very thin. Another problem with materials like Nafion™ is that they are not dimensionally stable when used to conduct protons. Variations in water content of the membrane, which are inevitable during proton conduction, cause considerable shrinking and swelling. Electrochemical cells incorporating Nafion™ membranes must be designed to accommodate such shrinking and swelling.
Gore-Select™ is a composite perfluorinated material consisting of a homogeneously porous substrate filled with an ion-conducting material. U.S. Pat. No. 6,613,203 describes a membrane of this type. FIG. 2 schematically depicts a Gore-Select™ membrane 10 having a homogeneous substrate 12 filled with an ion-conducting material 14. Porous substrate 12 provides membrane 10 with some degree of structural integrity and dimensional stability, while ion-conducting filler 14 provides proton conductivity.
There remains a need for ion-conducting membranes for use in electrochemical applications, such as fuel cells, electrolysis cells, chlor-alkali plants and the like, which possess advantageous mechanical properties and desirably high ion conductivity.